1. Field of the Invention
The present invention relates to an interconnection structure and a fabrication process therefor and more particularly to an interconnection structure that improves a filling performance of an interconnect material in itself and a fabrication process for the interconnection structure.
2. Description of the Related Art
Aluminum alloy has widely been adopted as an interconnect material for LSI. As a demand for miniaturization of LSI and its fast operating speed has been piled up, however, ensuring sufficient performances such as high reliability and low resistivity has increasingly become difficult with aluminum alloy interconnection. To cope with such a situation, a copper interconnection technique, because copper is excellent in electromigration resistance as compared with aluminum alloy, draws attention and is on the move toward practical use.
In formation of copper interconnects, since dry etching of copper generally is not easy, a fabrication process by means of trench interconnection is regarded as prospective. The trench interconnects are formed in a procedure such that predetermined trenches are in advance formed in an interlayer insulating film made of silicon oxide or the like, an interconnect material is filled into the trenches and thereafter, unnecessary portion of the interconnect material is removed by a chemical, mechanical polishing (hereinafter referred to as CMP, which is an abbreviation of Chemical Mechanical Polishing) or the like.
In the above described trench interconnect technique, various processes or methods for filling an interconnect material into the interior of contact holes and trenches have been studied, such as an electrolytic plating process, a chemical vapor deposition (hereinafter referred to as CVD, which is an abbreviation of Chemical Vapor Deposition) method, a reflow method and a high pressure reflow method, the latter two of which are effected after sputtering. Among them, an electrolytic plating process for copper which process has a high filling ability into the interiors of fine contact holes or trenches is regarded as an especially important technique.
An example of a process in which copper is filled into trenches and contact holes by means of the electrolytic plating method will be described with reference to FIG. 4.
As shown in FIG. 4(1), after a predetermined element (not shown) is formed on a semiconductor substrate (not shown), an interlayer insulating film 111 is formed thereon and an lower layer interconnect 112 constituted of trench interconnection is further formed in the inter layer insulating film 111. An interlayer insulating film 113, between upper and lower layer interconnection, which covers the lower layer interconnect 112, is formed on the interlayer insulating film 111 with an oxide silicon film 114 and a silicon nitride film 115. Then, part of a contact hole 116 is formed in the silicon oxide film 114 and thereafter, an interlayer insulating film 117 is further formed. A trench 118, in which a trench interconnect is formed, is formed in the interlayer insulating film 117 and thereafter, the contact hole 116 is completed in the interlayer insulating film 113 so as to reach the lower layer interconnect 112. Subsequently, titanium nitride is deposited on inner walls of the trench 118 and a contact hole 116 to form a barrier metal layer 119 of 70 nm in thickness. The barrier metal layer 119 has a blocking function against diffusion of copper into the interlayer insulating films made of silicon oxide or the like.
Then, as shown in FIG. 4(2), a seed layer 120 made of copper is deposited on the barrier layer 119 to a thickness of 100 nm by sputtering. The seed layer 120 functions as a seed in electrolytic plating conducted in a later step. Then, as shown in FIG. 4(3), copper 121 is filled into the interiors of the contact hole 116 and the trench 118 by means of an electrolytic plating process.
In the above described process, in order to realize good filling of copper 121, it is important to form the seed layer 120 in the respective interiors of the trench 118 and contact hole 116, so to speak, in a conformal manner. Furthermore, it is also significant to constantly supply a fresh plating solution into the respective interiors of the trench 118 and the contact hole 116.
However, decrease in interconnect line dimensions has progressed and a high aspect ratio, as a whole, of a trench and a contact hole combined has been a reality in a filling process in which the trench and the contact hole are simultaneously filled, by which a sputtering method has been hard to achieve a sufficient step coverage performance of a copper seed layer. In such circumstances, it is also difficult to constantly supply a fresh plating solution into respective interiors of a trench and a contact hole. In cases where step coverage of a copper seed layer is insufficient, or a fresh plating solution cannot sufficiently be supplied into the respective interiors of a trench and a contact hole in a continuous manner, voids or the like are produced in the respective interiors of a trench and a contact hole, thereby leading to poor filling. As a result, defective connection is provoked between a plug made of copper filled in the though-hole and a lower layer interconnect.
To cope with such inconvenience, methods have been studied of improving step coverage of copper in sputtering through improvement of an apparatus, For example, a long-distance sputtering in which a distance between a target and a substrate is increased and an ionization sputtering in which sputtered atoms are ionized to increase a fraction of vertical directionality in movement. A low-melting point metal material such as copper, however, is essentially poor in step coverage performance as compared with a high-melting point metal material such as tantalum nitride or titanium nitride and therefore, improvement of step coverage by the above described methods is not necessarily attained with ease. In a plating solution, on the other hand, while a process has been adopted in which wettability of the plating solution is increased by mixing an additive such as a surfactant into the plating solution, there is a limitation on improvement by increase in the wettability and a sufficient improvement of the filling characteristic of copper cannot be expected only by use of an additive.
Besides, in a structure in which a barrier metal layer is formed, since the barrier metal layer is formed between a lower layer interconnect and a copper plug formed in a contact hole, a contact resistance between the copper plug formed in the contact hole and the lower layer interconnect is increased.
The present invention is directed to an interconnection structure developed in order to solve the above described problem and a fabrication process therefor and the interconnection structure comprises: a recess, which is connected onto a conductive material mass formed in an insulating film, and which is formed in the insulating film, including a contact hole, or a trench, or a trench and a contact hole formed at a bottom of the trench; a barrier metal layer formed on side walls of the recess; and a metal material mass filled in the interior of the recess. The metal material mass is made of a metal that is repeatedly filled into the interior of the recess a plurality of times and the metal material mass is directly connected with the conductive material mass.
In the above described interconnection structure, a barrier metal layer is formed only on side walls of a recess and a lower portion of the metal material mass is directly connected to the conductive material mass at a bottom of the recess without a barrier layer interposed therebetween. Hence, a contact resistance between the conductive material mass and the lower portion of the metal material mass is low. Besides, the metal material mass is made of a metal that is repeatedly filled into the interior of the recess a plurality of times, which makes it possible to perform filling the metal into the interior of the recess by first effecting electroless plating, then followed by electrolytic plating. That is, in a case of a recess with a high aspect ratio, a first metal material mass is formed by means of electroless plating which has a high filling effectiveness, when an aspect ratio of the recess is lowered by formation of the first metal material mass, a seed layer is deposited, for example, by sputtering in a high step coverage state and thereafter, a second metal material mass can be formed by electrolytic plating whose deposition speed is high.
A first fabrication process for an interconnection structure comprises the steps of: forming a barrier metal layer on inner walls of a recess, for example, including a contact hole, a trench, or a trench and a contact hole formed at a bottom of the trench, which recess is connected onto a conductive material mass formed in an insulating film, and which recess is formed in the insulating film; exposing a conductive material mass at the bottom of the recess by removing the barrier metal layer formed at the bottom of the recess; forming a lower portion of a metal material mass by filling a metal into the interior of the recess; forming a seed layer by means of an electrolytic plating method on inner walls upward from the lower portion of the metal material mass; and forming an upper portion of the metal material mass by filling the metal into the interior of the recess by means of the electrolytic plating method.
In the first fabrication process for the interconnection structure, the lower portion of the metal material mass is formed after removing a portion, formed on the bottom, of the barrier metal layer formed in the recess. Hence, the lower portion of the metal material mass can be formed by electroless plating. Since an aspect ratio of the recess is reduced due to the presence of the lower portion of the metal material mass, step coverage of a seed layer by means of electrolytic plating, which is performed in a later step, is improved. As a result, the metal is satisfactorily filled into the recess by means of electrolytic plating, whereby no void occurs.
A second fabrication process for an interconnection structure comprises the steps of: forming a lower portion of a metal material mass by deposition of a metal into the interior of a recess, for example, including a contact hole, a trench, or a trench and a contact hole formed at a bottom of the trench, which recess is connected onto a conductive material mass formed in an insulating film, and which recess is formed in the insulating film; forming a barrier layer on inner walls of the recesses upward from the lower portion of the metal material mass; forming a seed layer by means of an electrolytic plating method on the inner walls of the recess; and forming an upper portion of the metal material mass by filling the metal into the interior of the recess by means of the electrolytic plating method.
In the above described second fabrication process, since an aspect ratio of the recess is reduced by the presence of the lower portion of the metal material mass formed by deposition of the metal into the interior of the recess, step coverage of a seed layer by electrolytic plating, which is performed in a later step, is improved. As a result, the metal is satisfactorily filled into the recess by electrolytic plating, whereby no void occurs.